If you store your bicycle in a public spot or in a room shared with other people than those in your household, you should always attach your bicycle by the frame to a fixed point with a certified lock.
If you store your bicycle in a locked, private, spot such as your basement, garage or own property, as long as you and the members of your household are the only people with access, it's not mandatory to attach it.
Note: two bicycles can also be attached with the same lock, as long as you follow the above-mentioned instructions.